Andreas Goldbach

Nanoscale heterogeneity in alkyl-methylimidazolium bromide ionic liquids

High-energy x-ray diffraction measurements and atomistic molecular dynamics (AMD) numerical simulations have been carried out on 1-alkyl-3-methylimidazolium bromide ionic liquids, C(n)mimBr, with n = 2, 4, and 6. Excellent agreement between experiment and simulation is obtained, including the region of the low-Q peak that has proved problematic in previous work in the literature. In the partial structure analysis of the AMD results, a distinct peak develops at the leading edge of the ring-ring pair distribution function and shifts to lower r with increasing alkyl chain length, indicating that the preferential parallel and antiparallel alignment of neighboring cation rings plays a larger role with increasing chain length. The ring-ring, anion-anion, and ring-anion partial structure factors are dominated by strong charge-ordering peaks around 1.1 Å(-1), corresponding to a distance between neighboring polar entities of D(2) = 5.7 Å. In contrast, the tail-tail S(Q) is dominated by the low-Q peak that rises and moves to lower Q with increasing chain length; the length scale of this structural heterogeneity D(1) increases from about 10 Å in C(2)mimBr to 14.3 Å in C(4)mimBr and 18.8 Å in C(6)mimBr. Both the length scale of the structural heterogeneity and its anomalous temperature dependence in the C(n)mimBr liquids studied here show considerable similarity to results in the literature for C(n)mimPF(6) liquids, indicating a remarkable insensitivity to the form and size of the anion. Our results are consistent with the concept of nanoscale heterogeneity with small, crystal-like moieties.

Structure of a Prototypic Ionic Liquid: Ethyl-methylimidazolium Bromide

High-energy X-ray diffraction measurements have been carried out on 1-ethyl-3-methylimidazolium bromide and complemented with molecular-dynamics simulations. Because the structure of the corresponding crystal is known, both the liquid and the crystal phases are simulated numerically. The liquid structure factor is dominated by an intense peak at 1.7 A(-1), associated mainly with the packing of the anions around the large cations. Analysis of the real-space correlations of the liquid shows that the Br(-) ions are distributed more symmetrically around the cation ring and move closer to the ring atoms compared with the crystal. Although the distribution of the anions around the cation in the first coordination shell of the liquid exhibits clear analogies with the crystal, the cation-cation partial distribution function of the liquid shows a significant component with lower distances between ring centers, with some pairs coming as close as 3.5 A in either parallel or antiparallel configurations. Finally, the presence of topological short-range order and charge ordering in the liquid is clearly demonstrated.

Permeation hysteresis in PdCu membranes

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H 2 at 823 K reveal retardation of the fcc(H) --> bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (1.3 +/- 0.2) mol.m (-2).s (-1) exp[(-11.1 +/- 0.6) kJ.mol (-1)/( RT)] and that in the fcc single phase regime above 873 K by J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].

CO2 decomposition over Pd membrane surfaces.

The interaction of pure CO 2 with a 3 microm thin, supported Pd membrane has been investigated between 473 and 773 K. Diagnostic H 2 permeation measurements indicate a reduction of the H 2 flux after CO 2 exposure at the lower and upper ends of this temperature range. Temperature-programmed oxidation and desorption in combination with scanning electron microscopy analyses reveal the dissociation of CO 2 above 523 K, yielding molecularly and/or dissociatively adsorbed CO below 623 K and nanoscopic carbon deposits above 723 K. CO 2 is obviously not inert over Pd surfaces at practical Pd membrane operation temperatures but could be kinetically stabilized in a narrow temperature window around 673 K.

Structural and Permeation Kinetic Correlations in PdCuAg Membranes

Addition of Ag is a promising way to enhance the H2 permeability of sulfur-tolerant PdCu membranes for cleanup of coal-derived hydrogen. We investigated a series of PdCuAg membranes with at least 70 atom % Pd to elucidate the interdependence between alloy structure and H2 permeability. Membranes were prepared via sequential electroless plating of Pd, Ag, and Cu onto ceramic microfiltration membranes and subsequent alloying at elevated temperatures. Alloy formation was complicated by a wide miscibility gap in the PdCuAg phase diagram at the practically feasible operation temperatures. X-ray diffraction showed that the lattice constants of the fully alloyed ternary alloys obey Vegards law closely. In general, H2 permeation rates increased with increasing Ag and decreasing Cu content of the membranes in the investigated temperature range. Detailed examination of the permeation kinetics revealed compensation between activation energy and pre-exponential factor of the corresponding H2 permeation laws. The origin of this effect is discussed. Further analysis showed that the activation energy for H2 permeation decreases overall with increasing lattice constant of the ternary alloy. The combination of these correlations results in a structure-function relationship that will facilitate rational design of PdCuAg membranes.

Approach to band gap alignment in confined semiconductors

Se has been sorbed into the pore system of Cu2+-ion exchanged Y zeolite to investigate the effect of transition metal countercations on the structure and electronic properties of the incorporated semiconductor. Results from anomalous x-ray scattering experiments at the Se K-absorption edge and Raman measurements show the formation of isolated, strongly disordered Sex chains (x→∞) with an intrachain bond length of 2.39 A. The difference pair correlation function TSe(r) exhibits a short-range pair correlation at 3.30 A, which is attributed to specific Cu2+-Sex interactions. The optical spectrum of the composite is characterized by a strong absorption band around 400 nm with a shoulder at 450 nm and an optical band gap of 2.09 eV. The strong Cu2+-Sex correlation is explained by charge transfer from the Sex chains to the cations, and the formation of band gap states is postulated to account for the unusually long intrachain bond length and the remarkably small band gap.

In situ characterization of catalysts and membranes in a microchannel under high-temperature water gas shift reaction conditions

Microreactor technology with high heat transfer in combination with stable catalysts is a very attractive approach for reactions involving major heat effects such as methane steam reforming and to some extent, also the high temperature water gas shift (WGS) reaction. For this study Rh/ceria catalysts and an ultrathin hydrogen selective membrane were characterized in situ in a microreactor specially designed for x-ray absorption spectroscopic measurements under WGS conditions. The results of these experiments can serve as a basis for further development of the catalysts and membranes.

Flame made ceria supported noble metal catalysts for efficient H2 production via the water gas shift reaction

Rh/ceria catalysts were synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions. These catalysts show a high specific surface area due to a high degree of nanocrystallinity. X-ray absorption spectroscopy (XAS) unraveled the formation of small Rh particles under WGS reaction conditions. The catalytic activity was examined at atmospheric pressure by measuring CO conversion as a function of temperature. Some methane formation was observed above 310°C.